This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved apparatus for supporting and maintaining the transverse alignment of a moving photoconductive belt.
In an electrophotographic printing machine, a photoconductive belt is charged to a substantially uniform potential so as to sensitize the surface thereof. Thereafter, the charged portion of the photoconductive belt is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive belt selectively discharges the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive belt corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive belt, the latent image is developed by bringing a developer mix into contact therewith. The developer mix comprises toner particles adhering triboelectrically to carrier granules. These toner particles are attracted from the carrier granules to the latent image forming a toner powder image thereon. The toner powder image is then transferred to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, and has been further amplified and described by many related patents in the art.
It is evident that the lateral alignment of the photoconductive belt is critical and must be controlled within prescribed tolerances. This is due to the fact that the photoconductive belt passes through many processing stations during the printing operation. As the belt passes through each of these processing stations, the location of the latent image must be precisely defined to optimize copy quality. Therefore, lateral movement of the photoconductive belt should be minimized so that the belt moves in a pre-determined path.
If the photoconductive belt was perfectly constructed and entrained about perfectly cylindrical rollers mounted and secured in an exactly parallel relationship with one another, there would be no lateral skewing of the belt. However, in actual practice, this is not feasible. Thus, the photoconductive belt must be tracked or controlled to regulate its lateral position. Existing methods of controlling the lateral movement of the photoconductive belt comprises various forms of crowned rollers, flanged rollers, and electrical servo systems. However, systems of this type frequently produce high local stresses resulting in damage to the sensitive photoconductive belt.
Accordingly, it is a primary object of the present invention to improve the support and lateral alignment of the photoconductive belt employed in an electrophotographic printing machine.